The present invention relates to a flotation riser which extends from a subsea location to a floating platform and is provided with a flotation structure and flotation system which allows full control of the buoyancy of the riser. In such risers it is often necessary because of severe weather to move the floating platform and thus, the riser must be disconnected from the floating platform at a level below the water surface so as not to present a hazard to shipping and the riser should have sufficient buoyancy to be free-standing against water currents so that it is not destroyed by excessive angular motion while disconnected from the platform. The time necessary to release a riser and cause it to be buoyant is limited in emergency conditions. To supply the air necessary to allow the riser to have sufficient buoyancy when it is discovered from the floating platform it is either necessary to have a high capacity air compressor or a smaller compressor with large capacity storage tanks on the floating platform. Neither of these alternatives is totally acceptable since space and weight at the platform are at a premium and the high cost of a high capacity air compressor.
In such risers it has been proposed previously to provide a riser and to surround the riser with a series of chambers with the riser forming the interior of the chamber. An example of this prior art structure is the B. J. Watkins U.S. Pat. No. 3,858,401. This patent discloses the riser flotation chambers surrounding the riser and a float control to control the amount of water and gas in each chamber and thereby control the buoyancy of each chamber. The W. J. Hayes et al U.S. Pat. No. 3,330,340 discloses another buoyancy structure for a marine conductor which includes sleeves extending across the individual flotation chambers and through which small diameter pipe strings can be contained and be freely insertable. U.S. Pat. Nos. 3,354,951 and 3,522,709 disclose other types of prior art buoyant marine conductors or risers.